Caption: Stem cells. Coloured scanning electron micrograph (SEM) of a cluster of human embryonic stem cells (HESC). HESCs are pluripotent, they are able to differentiate into any of the 200 cell types in the human body. The type of cell they mature into depends upon the biochemical signals received by the immature cells. This ability makes HESCs a potential source of cells to repair damaged tissue in diseases such as Parkinson's and insulin- dependent diabetes. HESCs could also be used to grow organs for transplant that would not be rejected. However, research using HESCs is controversial as it requires the destruction of an embryo. Magnification: x13,600 when printed at 10 centimetres wide.